This invention relates, in general, to scanning mass spectrometers and, in particular, to a scanning mass spectrometer having an angled focal plane with an electro-optical ion detector to enable detection of ions over a wide mass range, both economically and with decreased analysis time.
FIG. 1 shows an artist rendition of a prior art double focus scanning mass spectrometer 10 comprising a sample inlet 12 having an ion source 14 with an ionizing region 16 in which sample gas molecules are subjected to an electron beam 20 to form an ion beam 22. The ion beam 22 is directed through an electrostatic sector 24 and a magnetic sector 26. Ions traveling through the magnetic sector are scanned as at 30 and those of a selected mass are focused at focal point 32 and directed through a narrow slit 34 to an electron multiplier 36.
FIG. 1 also shows schematically how a mass spectrometer is made compact by folding the ion path and by using magnetic scanning to limit the necessary size of the magnetic sector and also shows how the pressure is reduced in the instrument by turbo-pump 40 and ion pump 42.
In this instrument, ions of slightly higher and lower mass focus along a nearly straight line locus, i.e., focal plane 44, which passes through the axial focal point 32 and which lies at an angle to the axis 34, viz, 37.4 degrees. This focal plane 44 which lies at an angle is hereafter called an angled focal plane and illustrated in phantom in FIGS. 1a and 1b. The preciseness of the angle of this angled focal plane is a function of the geometry of the instrument, i.e., the 90 degree electrostatic sector and the 90 degree magnetic sector. The effect of these sectors on the ions of different masses is discussed at length in an article entitled "Ion Optics" by Dr. Heinz Ewald and Dr. Heinrich Hintenberger Max Planck-Institut fuer Chemie, Mainz "Methoden und Anwendungen der Massenspektroskopie" Verlag Chemie G.m.b.H. Weinheim/Bergstrasse (1953) P. 52-91 and in an article entitled "Electro-Optical Detectors In Mass Spectrometry Simultaneous Monitoring of All Ions over Wide Mass Ranges" by Boettger, Giffin & Norris, ACS Symposium Series, 102:291-318.
As can be seen in FIG. 1b, because of the geometry of the instrument, the ions which have slightly higher and lower mass, identified therein as 32a, 32b, 32c and 32d by way of example, are ordinarily not detectable since they are blocked from entering the electron multiplier 36 and will continue to be blocked and undetected until the magnitude of the magnetic field is increased or decreased (also called stepped or adjusted) to bring these ions into focus and directed to the electron multiplier 36. When the magnetic field intensity is changed, time is required for this change and for subsequent stabilization to the new value. Typically for toxic gas monitoring, the stepping and stabilization times cause an increase in cycle time so that the total acquisition time for 50 ports may be up to 3 hours. This is because a typical system may be required to measure up to 40 different gases, each one requiring 15 discrete magnetic field settings, and these measurements repeated for each of 50 ports.
This invention provides a means for detecting ions along this angled focal plane using an electro-optical ion detector with the image intensifier plate of a channel electron multiplier assembly placed at the angled focal plane giving essentially simultaneous analysis of a limited range of ion masses, but switchable over several ranges for detection over a wide mass range. Thus, changing of the magnetic field is required less often for detection of a wide range of ion masses. For example, utilizing this invention, the magnetic field would need to be readjusted only 6 times to detect mass 40 to 400 versus up to 600 times if the mass spectrometer is used without the electro-optical detector.
Electro-optical detectors in mass spectrometers have been used before, but the image intensifier plate was placed normal to the magnetic sector exit port or placed at a very small angle. These prior art instruments were designed to have a long focal plane, a large image intensifier, and a correspondingly long fiberoptic assembly (window).
These channel electron multiplier assemblies are also called microchannel plate assemblies and the image intensifier plate is also called a microchannel electron multiplier, a microchannel plate, a photo cathode plate, or simply an image plate.
FIGS. 2a and 2b are examples of such prior art detectors which are more fully explained in the Boettger et al article, supra.
In FIG. 2a, the channel electron multiplier assembly 50 is connected to the output of a magnetic sector 26a of a non-scanning type mass spectrometer, also called a spectrograph type mass spectrometer, and the image plate 52 is coupled to a fiberoptic window 54 and to a photodiode array 56. The image plate 52 is located along the focal plane of the magnetic sector 26, such as in FIG. 1, and a phosphor coating 60 on the entrance ends of the fiberoptic window 54 transforms the electron energy from the image plate 52 into light energy which, in turn, is transferred to the photodiode array 56 outside the vacuum envelope represented by the vacuum flange 62. The image plate 52 comprised a linear array of 14 one inch long microchannel electron multipliers and the photodiode array 56 comprised 14 one inch long photodiode detectors, each of which contained a linear array of 1024 diodes. This prior art photo-diode array 56 had a number of difficulties and limitations including; (1) the detector had to be quite large in order to cover a large mass range which means extremely expensive components and associated fiberoptics, (2) a large vacuum envelope with corresponding cost penalties and (3) the photo diode array had to be extremely large which also meant a greatly increased cost.
FIG. 2b shows a fiberoptic window 70 coupled to an image plate 72 which is aligned with a focal plane 74 at an angle to the exit axis 76 of the ion beam 22. The maximum angle between the focal plane 74 and the image plate 72 is limited by the ability of the fibers in the fiberoptic window 70 to accept the light impinging upon the ends of the fibers. If the angle is too large, then the efficiency of light transfer is very low. The fibers in the fiberoptic window 70 must be fused together into one monolithic piece of glass; otherwise gases trapped between the fibers become extremely difficult to remove when attempting to produce high vacuum inside the analyzer.
Still another illustration of the prior art is shown in FIGS. 3a-3c and more fully explained in an article entitled "Microchannel Plate Detectors" by Wiza, Nuclear Instruments & Methods, Vol. 162, 1979, pages 587 to 601. As shown in FIG. 3a, the microchannel or image plate 80 is a circular glass structure with a plurality of channels 82 in which straight channel electron multipliers 84, as shown in FIG. 3b, are placed and optically coupled to a fiber optic window 86 the ends of which are coated with a phosphor to form a screen 90 as shown in FIG. 3c. The components shown in FIG. 3c are suitably clamped together by bolts 92, to form a channel electron multiplier assembly 94 which operates the same as the channel electronmultiplier assembly of FIG. 2a. Assembly 94 is commercially available and used in the present invention as will be described in full detail hereinafter.
It is an object of this invention, therefore, to provide a double focusing mass spectrometer with a electro-optical ion detector including a channel electron multiplier assembly of the type described with the image surface of the channel electron multiplier plate placed along an angled focal plane, a photodiode array, and a fiberoptic assembly of a special shape so that a limited range of ion masses can be detected essentially simultaneously so that the magnetic field is stepped fewer times thus greatly increasing measurement speed.